1. Field of Invention
The present invention relates assemblies of two or more thermoplastic components, and in particular, to a method for connecting two or more thermoplastic components together by an improved fusion-welding technique and to an improved fusion-welded assembly.
2. Description of the Related Art
One of the notable advantages of thermoplastic materials, such as, for example polyethylene, perfluoroalkoxy, hexafluoropropylene, polypropylene, and polyvinyldifluoride, in fabricating many industrial products is that, in many instances, components made of such materials can be welded together to form assemblies. There are many methods known in the plastic arts for welding thermoplastic components together. For example, the filter cartridge industry has used various welding techniques to fabricate filter cartridges, comprising thermoplastic housings and thermoplastic-encased filter elements, including spin-welding, contact-welding, injection-welding, and fusion-welding. In fusion welding, the mating surfaces of two or more thermoplastic components are heated to a molten state, or "wetted", for example, by an infrared (IR) heating device, before the components are mated together. After heating the mating surfaces, but before the surfaces cool, the components are pressed together so that the wetted mating surfaces of the components contact one another. As the mating surfaces cool, they fuse together forming an intermingled thermoplastic interface, thus securing the components to one another.
Of the various thermoplastic welding techniques known, many skilled in the art prefer IR fusion-welding methods because of certain advantages provided by such methods. One known advantage of IR fusion-welding is that it produces a stronger bond between welded components than that produced by other thermoplastic welding techniques, because infrared radiation heats the thermoplastic materials internally and penetrates deeper into the thermoplastic materials. Accordingly, the resulting fusion bond between the welded components provides more than a mere adhesion of the component surfaces. Another appreciated advantage of IR fusion-welding over other thermoplastic welding techniques is that IR fusion-welding methods do not produce undesirable by-products or debris, which is especially important when a production environment is required to be clean. For example, many skilled in the art of filter cartridge manufacturing prefer IR fusion-welding over alternative welding techniques, such as spin-welding, in which debris is created while generating the necessary frictional heat through spinning of the thermoplastic components relative to one another. IR fusion-welding may also be preferred over contact-welding methods, because some contact-welding applications tend to melt the thermoplastic components excessively.
Despite its known and appreciated advantages, including, but not limited to the above, the inventors of the present invention have found the use of conventional IR fusion-welding techniques to be either impracticable or inefficient in certain situations. For instance, the inventors have found conventional IR fusion-welding methods to be unsuitable when the thermoplastic components to be welded have complex non-coplanar geometries which make it difficult, if not impossible, to apply plastic-melting radiation to only the mating surface of one or more of the thermoplastic components to be welded together without applying too much radiation to other surfaces of the component. This situation arises when the shape of a thermoplastic component to be welded is such that when the mating surface of the component is presented to the IR heating device for wetting the mating surface prior to welding, another surface of the component which forms no part of the mating surface is actually closer to the IR heating device than the mating surface. Thus, the closer surface is heated, and perhaps overheated and damaged, before the mating surface is sufficiently wetted.
FIG. 6 illustrates one such example. A first thermoplastic member shown in FIG. 6 is a cylindrical member 22 having a sidewall 24 and radial endwall 26 with a bore 28 formed in its center. A second thermoplastic member is an insert 10 having a cylindrical body 12, sized and shaped to fit within the bore 28, with a peripheral, radially-extending flange 14. The insert may have a closed bottom surface 30 as shown, or the insert may have an open end. A mating surface 16 of the insert is defined on the underside of the flange 14. The mating surface 16 contacts and mates with a portion of the endwall 26 of cylindrical member 22 surrounding the bore 28 when the cylindrical body 12 of the insert 10 is inserted into the bore 28 of cylindrical member 22.
To weld the insert 10 to the cylindrical member 22 by a conventional IR fusion-welding technique, the insert 10 would be first aligned coaxially with the bore 28 of the cylindrical member 22 in a spaced-apart relation with respect to the cylindrical member 22, and the insert 10 would be oriented as it would be disposed within the cylindrical member 22. An infrared heating device, such as an IR platen 20, would be positioned between the insert 10 and the cylindrical member 22, and the mating surface 16 and end wall 26 would be subjected to IR radiation until sufficiently wetted. Next, the IR platen 20 would be removed and the insert 10 would be inserted into the cylindrical member 22 until the mating surfaces 16 and 26 contact one another and fuse together as the thermoplastic cools.
Problems arise here, however, because of the non-coplanar geometry of the insert 10. The mating surface 16 of the insert 10, which needs to be heated, is recessed relative to a bottom surface 30 of the insert 10, which is actually closer to platen 20 than the mating surface 16. If the mating surfaces 26, 16 of the cylindrical member 22 and the insert 10 were to be irradiated, the end surface 30 of the insert 10 would melt before the mating surface 16 is sufficiently heated and wetted, thus rendering the mating surface 16 essentially inaccessible to the radiation of the IR platen 20. Therefore, IR fusion welding is not a practical option for welding such an insert into a cylindrical member. Thus, to attach the insert 10 into the cylindrical member 22, one skilled in the art would heretofore have resorted to other, less desirable welding techniques, such as spin-welding.
Other problems attendant to conventional IR fusion welding techniques and other thermoplastic welding techniques, such as spin welding, arise when a welded thermoplastic assembly includes more than two components welded together. After the first two components of the assembly are welded together, structural and geometrical interferences created by the two connected components can make it difficult to weld subsequent components to the assembly.
Accordingly, while IR fusion welding provides certain advantages over other thermoplastic welding techniques, geometrical or other limitations can make it impracticable to employ IR fusion welding to connect two or more thermoplastic components to form a thermoplastic assembly.